In known manner, such light is generated in the active layer of a light-amplifying structure through which there passes a pumping current, and said structure is included in a monocrystalline chip made of a semiconductor material having III-V type binary composition, i.e. gallium arsenide AsGa or indium phosphide InP. Such a chip is fabricated by epitaxially growing layers one on another from a substrate. In the description below and as is the common practice, it is assumed that the substrate constitutes the bottom portion of the chip. Conventionally, and advantageously, the substrate has N-type doping and the layers of the amplifying structure necessarily form an N I P sequence, with the top layers of the chip usually having P-type doping.
The optical cavity of the laser is constituted by a bottom mirror and by a top mirror formed respectively on a bottom face and on a top face of the chip. In a known disposition which is adopted in the context of various implementations of the present invention, the pumping current is delivered to the amplifying structure through the top mirror. The light to be emitted then escapes from the cavity through the bottom mirror. In the typical case of the top mirror occupying only a fraction of the horizontal surface area of the chip, and where there is only a short distance between the top mirror and the amplifying structure, that known disposition has the advantage of ensuring the required coincidence between two zones of the surface of said structure, i.e. the zone through which pumping current passes and the zone which is included in the optical cavity. However, passing the current through the top mirror presents a problem when making the mirror. The problem is due to the fact that the mirror must simultaneously present both very high reflectance, preferably at least 99.5% so as to enable optical oscillation to build up in the cavity, and flow electrical resistance, preferably less than 10.sup.-4 .OMEGA./cm.sup.2 if resistance is considered per unit area, and in any event the resistance must be small enough to allow the pumping current to flow, e.g. at a density of 10 kA/cm.sup.2, while avoiding troublesome losses and heating due to the Joule effect.
In a first known surface emitting semiconductor laser, sufficiently low electrical resistance is obtained by using a metal mirror. Unfortunately, reflectance then has a maximum of about 97% and as a general rule that is insufficient.
That is why, in a second known surface emitting semiconductor laser, a doped semiconductor Bragg mirror is used. Such a mirror is formed by superposing layers having alternating refractive indices. To enable their crystal lattices to match the lattice of the chip, these layers are conventionally constituted by alternating layers of indium-gallium arsenide-phosphide, and of indium phosphide for an indium phosphide chip. For the same reason, the layers are conventionally constituted by aluminum-gallium arsenide and by gallium arsenide with a gallium arsenide chip. They can be doped so as to obtain the desired low electrical resistance.
That second known laser is described in a communication by D. Babic et al., IPRM 95, "Transverse mode and polarization characteristics of double-fused 1.52 .mu.m vertical-cavity laser", pp. 773-776. Both mirrors in that laser are semiconductor Bragg mirrors. To enable the pumping current to pass through, the bottom mirror situated on the N side of the amplifying structure is N-type doped, while the top mirror situated on the P side of the structure is P-type doped. In that laser, the chip is based on indium phosphide to enable light to be delivered at a wavelength of 1520 nm. To limit light absorption by having mirrors of limited thickness, the mirrors are based on gallium arsenide. When fabricating such lasers, each mirror must be disposed on one face, e.g. a top face, of a baseplate of relatively thick indium phosphide. For that purpose, the mirror is initially fabricated by epitaxial deposition on a is relatively thick substrate of gallium arsenide. Thereafter, it is assembled to the indium phosphide baseplate by thermal fusing. The substrate reinforces the baseplate mechanically such that the opposite face of the baseplate, e.g. its bottom face, can then be etched to make it thinner. A second operation of the same type then makes it possible to deposit the second mirror so as to form an optical cavity of appropriate length, with said length being measured vertically, i.e. in the thickness direction of the baseplate. On this topic, reference may also be made to an article published by D. I. Babic, K. Streubel, R. P. Mirin, N. M. Yargalit, E. L. Hu, D. E. Mars, L. Yang, and K. Carey, in Photon. Techn. Lett., Vol. 7, 1225, 1995.
Document EP-A-0 709 939 (Hewlett Packard Co.) describes a third known surface emitting semiconductor laser. That laser is included in a matrix of lasers of the same type. An object of that document for that laser is that it should be controlled on the N-type doping side of its light amplifying semiconductor structure ("N drive", to facilitate control) while being formed on an N-type substrate (that of the matrix, for the purpose of avoiding harmful diffusion of zinc which would otherwise be included in the substrate to give it P-type conductivity). In spite of the fact that the amplifying structure is an N I P structure, i.e. the doping on the two sides of the structure is of opposite types, the object is achieved by a tunnel junction formed for that purpose in the chip and electrically interposed in series between the structure and the substrate. The junction is reverse-biased, but the-two layers constituting it receive doping at sufficient concentration to enable it to transmit the pumping current by the tunnel effect. The mirrors situated on the N-doped and P-doped sides of the amplifying structure are Bragg mirrors having respective N and P types of doping. The tunnel junction is formed between the mirror having P-type doping and the substrate.
Those second and third known lasers suffer in particular from the drawback of the semiconductor materials which constitute the top mirror absorbing the light of the laser to such an extent that the reflectance of the mirror is less than the desired value.
This drawback is a result of the fact that binary semiconductor materials with P doping present an absorption coefficient for said light which is greater than that of materials having N-type doping. The coefficient increases with increasing wavelength of the light.